The field of the invention is that of mobile telecommunications.
A mobile telecommunications operator generally divides a geographical expanse of coverage into zones, each of which he equips with a transmit/receive antenna controlled by an access network node.
When the antenna is multisector, the latter transmits and receives in several so-called main directions which each define an angular sector of coverage so that the union of all the sectors ensures the transmit and receive coverage of the zone equipped by the antenna.
In technologies using the multiplexing of a multitude of channels, other than that of frequency multiplexing, a single fixed frequency common to several antennas allows a mobile to travel from one zone to another without having to change frequency.
The UMTS technology according to which orthogonal spectrum spreading codes on the fixed frequency are allocated to each channel is for example known. To each zone there corresponds a base node, (node B) of the access network which then manages a cell in the case of a single-sector antenna or several cells in the case of a multisector antenna, each cell then corresponding to a sector of the zone. A radio network controller (RNC) supervises several base nodes. One distinguishes between the downlinks for the communication channels from the access network to the mobiles and the uplinks for the communication channels from the mobiles to the access network. On each downlink, a distinct scrambling code makes it possible to distinguish a cell, a distinct orthogonal variable spreading code (OVSF standing for orthogonal variable spreading factor) makes it possible to distinguish each service in a cell. On each uplink, a distinct scrambling code makes it possible to distinguish a mobile, a distinct OVSF code makes it possible to distinguish each service within a mobile. Different collections of codes are allocated to neighbouring cells so as to minimise the external interference between cells. For a given fixed frequency, the quantity of possible channels is limited by the number of codes available.
When a mobile exits one zone in order to enter another, the preservation of frequency permits a soft handover. When a mobile is in a soft handover state, it has available several distinctive codes of zones for listening to several base nodes, each base node having available at least one distinctive code of the mobile for listening to the mobile. By means of known techniques, the RNC then manages the downlinks to the mobile and the uplinks from the mobile in such a way as to ensure continuity of communication on handover from one zone to another.
With a multisector antenna, when a mobile exits a sector in order to enter another, the preservation of frequency permits a softer handover. When a mobile is in a softer handover state, it has available several distinctive codes of sectors for listening to the base node on the basis of several main directions of the antenna, the base node then listening to the mobile on the basis of these same main directions of the antenna. By means of known techniques, the base node then manages the downlinks to the mobile and the uplinks from the mobile in such a way as to ensure continuity of communication on handover from one sector to another.
A problem arises, however, when the mobile is moving within the vicinity of a border between sectors, possibly making return trips from one sector to another. There may then be an undue number of softer handovers which among other things have the drawback of greater consumption of radiating power induced by the transmission of the antenna along two main directions and possibly by a larger power called for from the mobile to compensate for the reception losses induced at the border by the angular distancing of the mobile from each of the main directions.
A problem also arises when the mobile is travelling both in the vicinity of a border between sectors and of a border between zones, on account of the competing of a soft handover and of a softer handover.